In recent years, the energy depletion, the environmental pollution and the like have issued as global problems, and therefore, hydrogen energy and fuel cells using the hydrogen energy have been increasingly used as a substitute of fossil fuel. A fuel cell is a device that transforms chemical energy of hydrogen into electrical energy. Since the fuel cell does not use an internal-combustion engine, there is no noise and vibration, and high efficiency can be achieved. Since pollutants are hardly generated, the fuel cell has come into the spotlight as a new energy source.
Fuel cells may be divided into a solid polymer fuel cell, a solid oxide fuel cell, a molten carbonate fuel cell, a phosphoric acid fuel cell, a direct methanol fuel cell and an alkaline fuel cell depending on the kind of an electrolyte. The fuel cells may be used as those for power generation, transportation, portable purposes, and the like.
In the solid polymer fuel cell (polymer electrolyte membrane fuel cell, or PEMFC), a solid polymer membrane is used as an electrolyte, and hence the solid polymer fuel cell can operate at a normal temperature and pressure. Further, the solid polymer fuel cell has come into the spotlight as a power source because of a lower operating temperature of about 70 to 80° C., a short operating time and a high power density. Recently, a polymer fuel cell operable even at 100 to 150° C. has been under development.
FIG. 1 is a perspective view of a fuel cell including a stainless steel separator.
Referring to FIG. 1, a solid polymer fuel cell stack 100 includes a membrane electrode assembly 110 including an electrolyte, electrodes (an anode and a cathode) and a gasket for gas sealing, a separator 120 having a flow channel, and an end plate including inlets and outlets 130 and 140 of air and inlets and outlets 150 and 160 of hydrogen gas.
The separator 120 is generally formed of any one of graphite, carbon, Ti alloy, stainless steel and conductive plastic. Preferably, the separator is formed of the stainless steel. The stainless steel has low interfacial contact resistance, excellent corrosion resistance, high heat conductivity, low gas transmittance. The stainless steel also has excellent mechanical strength and formability at a thin plate. Thus, the stainless steel has an advantage in that the volume and weight of the fuel cell stack can be reduced.
However, the separator 120 formed of the stainless steel has a problem in that the interfacial contact resistance between a material surface of the separator and a membrane electrode assembly (MEA) layer may be increased by semiconducting characteristics of an passive film formed on the surface of the separator 120 under fuel cell operating conditions. Further, the separator 120 requires excellent corrosion resistance in the fuel cell operating environmental atmosphere having a strong acidic atmosphere.
In order to solve such a problem, U.S. Pat. No. 6,835,487B2 and Korean Patent No. 0488922 disclose a method for obtaining surface characteristics of a stainless steel to a desired level by regulating the mean surface roughness Ra that shows a surface roughness to be 001 to 1.0 μm and regulating the maximum height Ry to be 0.01 to 20 μm so as to decrease the surface contact resistance of the stainless steel to 100 mΩcm2 or lower. Here, the stainless steel contains Cr (16 to 45 wt %) and Mo (0.1 to 3.0 wt %) and additionally contains Ag (0.001 to 0.1 wt %). Japanese Laid-Open Publication No 2007-026694 discloses a method for obtaining surface characteristics of a stainless steel to a desired level by forming micro-pits of 0.01 to 1.0 μm on the entire surface of the stainless steel containing Cr and Mo. U.S. Pat. No. 6,379,476B1 discloses a method for preparing a ferritic stainless steel having a mean roughness of 0.06 to 5 μm by exposing carbide (carbide inclusion) and boride (boride inclusion) to a surface of the ferritic stainless steel. Here, the ferrite stainless steel contains 0.08% or more C for forming carbide. Japanese Laid-Open Publication No 2005-302713 discloses a technique for ensuring the locally calculated mean interval S=0.3 μm or less and the second-order mean-square slope Δq=0.05 or more in a stainless steel containing Cr (16 to 45 wt %) and Mo (0.1 to 5.0 wt %).
However, these methods are provided only for the purposes of decreasing contact resistance by regulating the surface roughness of the stainless steel, micro-pits or conductive inclusions. To this end, the surface roughness of the stainless steel should be strictly maintained. Therefore, productivity is lowered, and production cost is increased. Further, it is difficult to secure reproducibility. In these methods, a component containing Cr and Mo as essential elements is specified within a predetermined range, and Ag, C and B for forming other conductive inclusions are added as additional elements to the stainless steel. Therefore, the increase in preparation cost may be caused, and it is not necessary to ensure stability of contact resistance and elution resistance under the fuel cell operating condition (60 to 150° C.) in acidic environment.
Japanese Laid-Open Publication No 2004-149920 discloses a method for reducing contact resistance by regulating the Cr/Fe atomic ratio to be 1 or more in a stainless steel containing Cr (16 to 45 wt %) and Mo (0.1 to 5.0 wt %). Japanese Laid-Open Publication No 2008-091225 discloses a method for reducing contact resistance by forming micro-pits in a stainless steel containing Cr (16 to 45 wt %) and Mo (0.1 to 5.0 wt %) and by securing the Cr/Fe atomic ratio to be 4 or more.
However, these methods have difficulty in specifying a component containing Cr and Mo as essential elements and stably securing low interfacial contact resistance without ensuring a strict control process of a passive film even under conditions having various surface roughnesses. Further, it is not necessary to ensure stability of contact resistance and elution resistance under the fuel cell operating condition (60 to 150° C.) in an acidic environment.